Biological small angle x-ray scattering and diffraction (SAXS) provides structural and dynamic information from solutions or partially ordered arrays of biomolecules in physiologically relevant conditions, albeit at lower resolution (~7-10 or higher compared to that obtainable from crystallography or cryo-electron microscopy. SAXS is not only a powerful tool to study structures of large macromolecular assemblies in solution, but can also be used for time-resolved studies to investigate large conformational changes that occur during enzymatic catalysis, protein- protein interactions, and folding of macromolecules. Such studies are critical for biomedical research, as they provide basic mechanistic information that can be used to understand the effects of deleterious mutations, and can thereby inform drug development that ultimately impacts human health. The application of SAXS methods to problems in structural biology has seen tremendous growth in recent years, reflecting the need within the research community for experimental methods that can give structural and dynamic information on large multi-domain complexes. The explosive growth of SAXS has largely been enabled by technical advances that include high- brightness, low divergence X-ray sources, advances in X-ray optics, and X-ray detectors. As protein solutions scatter X-rays quite weakly, improved X-ray detectors are a critical enabling technology. The silicon pixel array X-ray detectors are ideally suited for SAXS, due to their zero read noise and photon counting technology, which open up new possibilities for SAXS applications in structural biology. This application is a request for a complete detector system for small angle X-ray scattering, consisting of a PILATUS3 X 1M Pixel Array Detector (PAD) manufactured by Dectris, Inc., and associated control, data acquisition, data handling and storage computing hardware. This system will be installed on beam line 4-2 (BL4-2) at the Stanford Synchrotron Radiation Lightsource (SSRL). The PILATUS3 1M PAD offers many advantages over the presently available detectors installed at the beam line including superior signal-to-noise, increased maximum frame rate, and larger dynamic range. The detector is well suited for size-exclusion chromatography coupled SAXS experiments, time-resolved, and high-throughput static SAXS measurements that will be needed to drive the forefront of BL4-2 research in structural biology on challenging biomedically important systems. It will maintain international competitiveness for NIH funded researchers.